The present invention relates generally to the field of analog to digital converters and more particularly to an analog to digital converter which can provide high resolution in a very short sampling period.
In the field of scanning spectrophotometers, a field in which the present invention is particularly useful, the output of a photomultiplier tube is measured periodically during a sampling period of approximately 6 milliseconds. As a consequence, the circuitry for measuring the analog output of the photomultiplier tube must respond very quickly.
In the field of analog to digital converters, numerous circuits have been developed for converting an unknown analog signal into a digital representation for the magnitude of the analog signal. One type of analog to digital (A/D) circuit is found in typical digital electronic meters such as multimeters, voltmeters, ammeters, ohmeters, and the like. Such meters typically have three or four display digits and provide resolution in the range of one part in two thousand. Such circuits are incapable of providing resolution of about one part in 65,000 within a 6 millisecond sample period which is required for the contemplated application of the present invention.
A second type of A/D converter is one where the unknown analog signal is applied to an integrator for a sample period. Periodically, during the sample period, a known charge is dumped from the integrator circuit capacitor. The number of charge dumps during the sample period is correlated to the amplitude of the unknown analog input. This type of A/D converter, however, is not capable of providing the desired resolution for the application contemplated for the present invention because insufficient sample time is available as about 1 microsecond is required for each current dump making possible resolution of about one part in six thousand during a 6 millisecond sample period.
A third type of A/D converter is one wherein a successive approximation circuit is utilized. This approach is useful for applications where the unknown input is constant during the period that the successive approximation is made. For scanning spectrophotometers where the present invention finds application, however, the analog output of the photomultiplier tube (PMT) is not constant through the period of time that the PMT output is available for measurement. Accordingly, successive approximation A/D converters are not usable in the application contemplated for the present invention.
Accordingly, it is the principal objective of the present invention to provide a high resolution analog to digital converter for measuring the magnitude of an unknown analog signal over a very short sampling period of time while concurrently averaging the signal.
It is a further objective of the present invention to provide a high resolution analog to digital converter for measuring the magnitude of an analog signal over a sample time of approximately 6 milliseconds yet providing resolution in the order of one part in 65,536.
It is a further objective of the present invention to provide a high resolution analog to digital converter utilizing a microprocessor and associated circuitry which, in applications such as scanning spectrophotometers, can also be utilized in the instrument itself for purposes other than analog to digital conversion.
It is yet a further objective of the present invention to provide a high resolution analog to digital converter for use in an instrument such as a scanning spectrophotometer utilizing a microprocessor wherein the additional hardware cost for producing the high resolution analog to digital converter is nominal compared to the cost of the remainder of the electronics utilized in the instrument.